Discharge lamp having an improved temperature homogeneity

ABSTRACT

A discharge lamp for dielectrically impeded discharges, having a discharge vessel filled with a discharge medium and having discharge electrodes which are at least partly separated from the discharge medium by a dielectric layer, wherein the discharge vessel is elongated at least in a longitudinal direction, characterized by a thermal device for controlling the heat transport into and out of the lamp nonhomogeneously in the longitudinal direction, which is designed such that in operation, the temperature in the lamp is made homogeneous in the longitudinal direction.

This application is a U.S. National Phase Application under 35 USC 371of International Application PCT/DE00/03519 (not published in English)filed Oct. 6, 2000.

FIELD OF THE INVENTION

The invention relates to a discharge lamp designed for dielectricallyimpeded discharges. Such a discharge lamp typically has a dischargevessel, which contains the discharge medium, conventionally xenon, ormore generally a gas mixture with a noble gas. For ignition andmaintenance of the discharges, electrodes are provided; discharge lampsdesigned for dielectrically impeded discharges are distinguished in thatat least the electrodes designed as anodes are separated from thedischarge medium by a dielectric layer, which can also be a wall of thedischarge vessel. It is also possible for all the electrodes to bedielectrically impeded, for instance to make the discharge lamp suitablefor a bipolar electrical power supply.

PRIOR ART

The fundamental physical events, technical properties and advantages aswell as possible applications in or of discharge lamps fordielectrically impeded discharges are understood here to be known. Therelevant literature can be referred to.

One essential performance characteristic for discharge lamps designedfor dielectrically impeded discharges, that is, so-called “silent ”discharge lamps, is the temporal and local homogeneity of the luminance.Special provisions for varying the distribution of the discharge in thedischarge space have this as the goal, in particular individuallocalizable discharge structures, by means of special electrodestructures that form preferential points for the discharge structures.Reference can be made for instance to German Patent Application DE 19636 965 A1. By localizing the single discharges by means of electrodestructures, optimized patterns in terms of the three-dimensionalarrangement can be specified, which fill the discharge space in such away that a favorable distribution of the luminance occurs. However,there is still a need for improvements to the temporal and localhomogeneity of the luminance, above all in discharge lamps withsignificant length in at least one direction, such as barlike lamps withone direction of longitudinal extension, and flat lamps with two suchdirections.

SUMMARY OF THE INVENTION

The invention is thus based on the technical problem of disclosing asilent discharge lamp that is improved in terms of the temporal andlocal homogeneity of the luminance.

According to the invention, this problem is solved by a discharge lampfor dielectrically impeded discharges, having a discharge vessel filledwith a discharge medium and having discharge electrodes which are atleast partly separated from the discharge-medium by a dielectric layer,wherein the discharge vessel is elongated at least in a longitudinaldirection, characterized by a thermal device for controlling the heattransport into and out of the lamp nonhomogeneously in the longitudinaldirection, which is designed such that in operation, the temperature inthe lamp is made homogeneous in the longitudinal direction.

The invention proceeds from the recognition that some nonhomogeneitiesin elongated silent discharge lamps occur only after time has elapsed inoperation. It was possible to conclude that there was a relationshipbetween the homogeneity of the temperature distribution in the dischargevessel and the homogeneity of the light projection. Evidently, since ahomogeneously equally distributed pressure prevails in a dischargevessel, a nonhomogeneity in the temperature distribution results in anonhomogeneous distribution of density of the discharge medium. Thedensity of the discharge medium in turn has an effect on the physics ofthe discharge. In this sense, evidently a nonhomogeneity in thetemperature resulting from the installation situation, the constructionof the lamp itself, external temperature nonhomogeneities or othercauses, is the cause of fluctuations in luminance over the at least onelongitudinal direction in which the lamp extends.

In particular, it has been found that these variations in luminance candevelop over the operating time, or in other words are generally linkedvia the purely local variation to a variation over time in the luminancedistribution in the initial phase of operation. Thus the lack oftemperature homogeneity is disadvantageous in two respects.

The general inventive concept is in the most general sense to exertinfluence on the temperature distribution by means of a thermal device,which controls the heat transport into and/or out of the lamp. Accordingto the invention, this involves not simply a powerful cooling device,for instance, with which in a sense the attempt is made, by means of asuitably generous design of the cooling apparatus, to impress itstemperature homogeneity on the lamps. Instead, the point of departurefor the invention is that the thermal device in turn nonhomogeneouslyinfluences the heat transport, specifically in a way that iscomplementary to the intrinsic temperature behavior of the lamp. This isintended to counteract the development of the nonhomogeneous temperatureprofile in the discharge lamp.

In principle, the term “thermal device ” used here covers any provisionsby which influence can be exerted on the heat transport. In particular,it includes the control of the heat transport to the outside out of thelamp and in the opposite direction. Accordingly cooling devices in themost general sense can be considered, that is, devices that reinforceand improve heat dissipation from the discharge lamp to the outside,insulating devices, that is, devices that reduce the heat transport,which in general means heat transport from the discharge lamp to theoutside, and finally also heating devices.

In very many cases, the intrinsic temperature behavior of the lamp, thatis, the nonhomogeneous temperature profile that occurs without thethermal device of the invention, is characterized in that peripheralregions of the discharge lamp are not heated as much during operation asmiddle regions. This can be due for instance to the fact that theperipheral regions, referred to the portion of the discharge spaceassigned to them, have a larger surface area and thus greater heatlosses. However, the invention also pertains to cases that areotherwise, for instance in which because of special mounting situations,the closeness of other components that produce heat, special dischargevessel geometries or otherwise, nonhomogeneous temperature profilesoccur.

Concrete possibilities for thermal devices of the invention are forinstance cooling bodies with cooling fins that are in thermal contactwith the discharge lamp; the presence of the cooling fins, their length,or the density of their disposition is nonhomogeneous in a way that isadapted to the intrinsic temperature behavior of the discharge lamp. Forinstance, the cooling fins may either be present only in the middle ofthe lamp, or be located closer together in the middle of the lamp, or bestretched out with a larger surface area. Nonhomogeneous cooling canalso occur from a mounting device which in the middle region of thedischarge lamp is coupled with good thermal conductivity, for instance,and acts as a cooling device by means of its own good thermalconductivity. This may for instance be a solid metal body. Naturally,both of these provisions may also be combined.

Another possibility is to insulate the peripheral regions of a dischargelamp thermally from the outside world, or else, by making thickerinsulators or other components with insulating properties that arepresent anyway, such as discharge vessel walls, to provide forreinforced insulation in the peripheral region. For details of thesevarious possibilities, see the exemplary embodiments in the furthercourse of this description.

In the introductory part of the description, conventional provisionshave already been mentioned with which influence can be exerted on thethree-dimensional distribution of individual discharge structures. Whatis essential is that the provisions proposed with this invention andthese conventional possibilities do not in any way preclude one anotherbut instead prove to reinforce one another. In this sense, the inventionis directed in particular to discharge lamps designed for the pulsedoperating process developed by the present Applicant. This pulsedoperating process assures the development of localizable individualdischarge structures. For details, reference may be made to the priorart, and in particular to International Patent Disclosure WO94/23442. Inparticular, the invention is thus also directed to a discharge lamp,designed according to the invention, with a ballast device provided forthe pulsed operating process.

One important application of silent discharge lamps is discharge lampswith an elongated barlike discharge vessel. In other words, they areelongated in only one longitudinal direction, and in the planeperpendicular to it are relatively small in cross section. Importantapplications of such “linear radiators ” are in the field of officeautomation (OA), for instance. They can be used in scanners, such as infax machines, electronic copiers, or in computer peripherals. They areequally suitable for conventional photocopiers. In this respect, itshould be stated that the invention relates not only to discharge lampsthat produce visible light but to UV radiators, for instance, as well.

In the field of these linear radiators, the invention is especiallyadvantageous in relatively powerful linear radiators, in whichexperience tells that the disadvantages that are overcome or at leastameliorated by the invention occur to an increased extent. Powerfullinear radiators can for instance have linear power densities of over0.3 W/cm.

However, the invention is equally suitable for use in flat radiators,that is, large-area, essentially two-dimensionally extended dischargelamps, for instance for lighting liquid crystal screens from behind. Insuch flat radiators as well, greater cooling of a middle region relativeto a peripheral region, or better insulation of the peripheral regionfrom the middle region, or heating of the peripheral region isadvantageous, that is, a thermal device along the lines of theinvention. In principle, the possibilities already described can bechosen, such as cooling fins; the cooling fins are disposedcorrespondingly nonhomogeneously not only in the longitudinal directionbut also in the transverse direction (that is, in the plane of the flatradiator). One example of this is a component part of the exemplaryembodiments that will be described hereinafter.

Another possibility that is also illustrated in the exemplaryembodiments has a generally flat metal sheet, which is in superficialthermal contact with the discharge vessel of the flat radiator. Recessesare provided in the metal sheet and define ribs that divide at least amiddle region of the sheet from a peripheral region, and optionally alsodefine a plurality of intermediate regions graduated from the middleregion toward the peripheral region. The middle region of the metalsheet can be cooled by being embodied as a cooling device itself, forinstance with cooling fins, or by being in thermal contact with acooling device. The ribs make it possible to vary the heat transportfrom the peripheral region into the cooled middle region, so that onceagain, nonhomogeneous control of the heat transport out of the lamp intothe metal sheet can be effected. The directly cooled middle region ofthe sheet will in fact cool the lamp more markedly than the outer regionor regions joined to it only via the ribs.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in further detail below in terms of variousexemplary embodiments. Characteristics disclosed can be essential to theinvention both individually or in combinations other than those shown.Individually, the drawings show:

FIG. 1, a schematic view of a first exemplary embodiment of a dischargelamp of the invention;

FIG. 2, a schematic view of a second exemplary embodiment of a dischargelamp of the invention;

FIG. 3, a cross section through the discharge lamp of FIG. 2;

FIG. 4, a schematic view of a third exemplary embodiment of a dischargelamp of the invention;

FIG. 5, a cross section through the discharge lamp of FIG. 4 in theperipheral region;

FIG. 6, a cross section through the discharge lamp of FIG. 4 in themiddle region;

FIG. 7, a schematic view of a fourth exemplary embodiment, namely a flatradiator according to the invention;

FIGS. 8A, 8B, 8C, sectional views through the exemplary embodiment ofFIG. 7;

FIG. 9, a fifth exemplary embodiment in the form of a flat radiator.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1, as the first exemplary embodiment, shows a silent discharge lampaccording to the invention, which essentially comprises an elongatedglass tube 1, closed on one end, which on its open end is closed by asuitable stopper 2. Details of this lamp with regard to the electrodestructure, the luminous coating, and the like will not be provided here,because they are not essential to the invention. However, German PatentDisclosure DE 197 18 395 can be referred for further details. Naturally,instead of the internal electrodes shown in this reference, one or moreexternal electrodes can be used.

In the longitudinal direction corresponding to the horizontal length interms of FIG. 1, the most homogeneous possible luminance of thedischarge lamp is to be attained. According to the invention, this isreinforced by a cooling body 3, which is joined thermally conductivelyto the lamp over practically the entire length of the lamp, or in otherwords rests over a large surface area on it or is clamped or glued toit. The cooling body 3 has many fins 4, forming cooling faces andoriented perpendicular to the longitudinal direction, which arestaggered parallel to one another. The cooling fins 4 are relativelysmall in size in an outer peripheral region 6, in the vicinity of theleft- and right-hand ends of the discharge lamp; that is, they protrudeonly a short distance from the base part, near the lamp, of the coolingbody 3. In a middle region 5, which corresponds to the center of thelamp, the cooling fins 4 are conversely embodied very much longer andtherefore exhibit a markedly greater cooling action. Between the middleregion 5 and the peripheral region 6, smooth transitions in terms of thelength of the cooling fin are provided.

As an alternative to this, FIG. 2 shows a second exemplary embodiment,in which a discharge lamp, corresponding to the view in FIG. 1, isprovided with a metal shell 7, which in turn is in good thermal contactwith the discharge lamp. FIG. 3 shows a cross section through the viewin FIG. 2, seen horizontally in terms of FIG. 2, specifically throughthe middle region 5 of FIG. 2. It can be seen that the metal shell 7embraces the discharge lamp on both sides, with approximately the upperthird of the discharge lamp, which is circular in cross section,remaining exposed so that it can emit light. Correspondingly, in theinterior of the discharge lamp, a reflective layer 9 is provided inareas complementary to the surrounding shell 7; see DE 197 18 395,already cited.

In FIG. 3, it can be seen in cross section that between the reflectivelayer 9 and the glass tube 1, opposed electrode strips 12 are provided,which are symmetrical to the opening of the lamp in the upper third. Atthese points, there is a spacing between the shell 7 and the glass tube1, within which spacing the shell 7 extends in a somewhat bulging form13 in the region of the electrode strips 12, with spacing from the glasstube 1. Somewhat above and somewhat below the electrode strips 12,however, the shell 7 again contacts the glass tube 1. This structure hasthe advantage of a low capacitive coupling between the shell 7 and theelectrode strips 12. It is not of essential significance to the thermalproperties, since in the remaining region of the shell 7, there is goodthermal contact with the lamp tube 1. The other exemplary embodimentscan also have comparable structures, which is not shown in furtherdetail.

In the middle region 5, the metal shell 7 has a foot 8, which in turn ismounted with good thermal conductivity on a base of no further interesthere, which however can act as a heat sink or heat buffer. As a result,in the middle region 5, the foot 8 carries heat to a greater extent outof the shell 7 and thus out of the discharge lamp than is the case inthe peripheral region 6, where there is no foot. The shell 7 with thefoot 8 in the middle region 5 thus forms a cooling device withnonhomogeneous action along the horizontal length of the discharge lamp,as is also the case in the first exemplary embodiment.

FIGS. 4, 5 and 6 show a further example. FIG. 5 shows a sectioncorresponding to FIG. 3 in perspective, but through a peripheral region6 of FIG. 4, while conversely FIG. 6 shows a corresponding sectionthrough the middle region 5. It can be seen that once again, a metalshell 10 is provided which is in good thermal contact with the dischargelamp, but in this case does not act as a cooling device but only as ashield against the electromagnetic radiation from the discharge lamp.This function is furthermore intrinsic to the metal shell 7 of FIG. 2 aswell. In both cases, the shells 7 and 10 are preferably grounded forthat purpose. In the third exemplary embodiment shown in FIG. 4,however, an additional thermal insulation 11 is provided around theshell 10 in the peripheral region 6, so that the middle region 5 canbetter dissipate the heat loss from the discharge to the outside thanthe peripheral region 6 can. This third exemplary embodiment accordinglyshows a corresponding thermal device, in the sense of a nonhomogeneousinsulation of the discharge lamp in the longitudinal direction.

Naturally the versions shown in the various exemplary embodiments canalso be combined; for instance, an insulation in the third exemplaryembodiment can be combined with cooling in accordance with the first orsecond exemplary embodiment. It is a common feature of all three casesthat the temperature homogeneity is markedly improved in thelongitudinal direction of the linear radiator. Because the gas densityinside the discharge lamp is largely homogeneous given the equaltemperature and largely homogeneous temperature, a very homogeneousdistribution of luminance is thus achieved as well.

FIG. 7 schematically shows an exemplary embodiment in the form of a flatradiator 14. The flat radiator 14 is shown here only as a flat,two-dimensionally extended plate, because its technical details arefamiliar to one skilled in the art from the prior art. Reference may bemade for instance to International Patent Disclosure WO98/43276.

A cooling body 15 is mounted on the flat radiator 14, on the sideopposite the light emission side, and covers the middle region of theflat radiator. FIGS. 8A, 8B, 8C show a section as indicated in FIG. 7,along the lines A, B and C, respectively (with a side view of thecooling body 15), through the flat radiator 14 and the cooling body 15of FIG. 7. It can be seen that as in the exemplary embodiments of FIG.1, the cooling body has cooling fins 16, extending parallel to andspaced apart from one another. The cooling fins 16 are designed suchthat they are highest in the middle region of the cooling body 15. Tothat end, they have a profile that extends upward from both ends to amaximum height in the middle; the maximum heights of the cooling fins inthe middle are staggered in such a way that the profile of an individualcooling fin 16 in FIG. 8A essentially corresponds to an envelope overthe maximum heights, visible in FIG. 8B, of the entire number of coolingfins 16. The overall result thus, as a result of the design of thecooling fins 16 and the central disposition of the cooling body 15 thatdoes not reach the periphery, is a nonhomogeneity of the cooling actionwith its focal point in the middle of the flat radiator 14.

Finally, FIG. 9 shows a final exemplary embodiment, which also pertainsto a flat radiator. The flat radiator is shown only schematically and isidentified overall by reference numeral 17. A metal sheet 18, largelycorresponding to the area of a discharge vessel, not shown in furtherdetail, is in good superficial thermal contact with the discharge vesselof the flat radiator 17 and has recesses shown in further detail in FIG.9. The recesses form various ribs 19, which connect a middle region 21of the metal sheet 18 to a peripheral region 20. In FIG. 9, anintermediate region is also provided between the middle region 21 andthe peripheral region 20, but it is not absolutely necessary. By meansof one or more such intermediate regions, the nonhomogeneity of thethermal influence on the discharge lamp can be made more uniform.

The middle region 21 is in contact with a cooling device, not furthershown, such as cooling fins. By means of the heat transport, limited bythe ribbed form of the metal sheet 18, between the middle region 21 andthe peripheral region 20, the middle region 21 is thus cooled very muchmore markedly than the only indirectly cooled peripheral region 20. Theintermediate region naturally assumes an intermediate position in termsof cooling.

Control of this heat transport can follow by means of the number and thelength, represented by a and b in FIG. 9, of the ribs.

This embodiment, like the others, can perform a function to shieldagainst electromagnetic radiation from the lamp or can be combined withthe shielding.

In the exemplary embodiments shown here, no relevant dependence of theluminance on the temperature is demonstrated, as long as the temperaturevariations are within the typical range of a few tens of Kelvins. Infact, thermal energies result but are not significant in comparison tothe definitive energies for the physics of the discharge. As long ascreating a nonhomogeneity of the gas density does not lead to anyheterogeneous discharge situation, not only can the local homogeneity beimproved as a result, but the startup behavior over time can bepractically avoided.

In the linear radiators shown here in particular, it is furthermorepossible, depending on the operating position, that is, above all invertical operation with an additional vertical temperaturenonhomogeneity, for a situation to arise in which, without theinvention, the ignition conditions inside a powerful elongated linearradiator would become so nonhomogeneous that the discharges would burnonly in the regions of lower gas density and thus over long freedistances. Since the heat loss is also concentrated in these regions,this mechanism has a self-reinforcing character. The invention offerseffective aid against it.

What is claimed is:
 1. A discharge lamp for dielectrically impededdischarges, having a discharge vessel (1, 2) filled with a dischargemedium and having discharge electrodes which are at least partlyseparated from the discharge medium by a dielectric layer, wherein thedischarge vessel (1, 2) is elongated at least in a longitudinaldirection, characterized by a thermal device (3, 4, 7, 8, 10, 11) forcontrolling the heat transport into and out of the lamp nonhomogeneouslyin the longitudinal direction, which is designed such that in operation,the temperature in the lamp is made homogeneous in the longitudinaldirection.
 2. The discharge lamp of claim 1, in which the nonhomogeneitydistinguishes a middle region (5) of the discharge lamp from aperipheral region (6).
 3. The discharge lamp of claim 1, in which thethermal device (3, 4, 7, 8, 10, 11) has a cooling device (3, 4, 7, 8).4. The discharge lamp of claim 3, in which cooling fins (4) areprovided, which are disposed nonhomogeneously in the longitudinaldirection in terms of their presence, their length and/or their density.5. The discharge lamp of claim 2, having a mounting device (8), which iscoupled to the middle region (5) in a manner providing good thermalconductivity.
 6. The discharge lamp of claim 1, in which the thermaldevice (10, 11) has an insulating device (11).
 7. The discharge lamp ofclaim 2, in which the peripheral region (6) is thermally insulated. 8.The discharge lamp of claim 1, in which the thermal device has a heatingdevice.
 9. The discharge lamp of claim 1, having a ballast devicedesigned for a pulsed operating process.
 10. The discharge lamp of claim1, in which the discharge electrodes have structures for defining thelocation of individual discharges.
 11. The discharge lamp of claim 1, inwhich the discharge vessel (1, 2) is elongated in barlike form.
 12. Thedischarge lamp of claim 11, which is designed for a linear power densityin the longitudinal direction of 0.3 W/cm or more.
 13. The dischargelamp of claim 11, which is designed for a photocopier or a scanner. 14.The discharge lamp of claim 1, which is embodied as a flat radiator (14,17).
 15. The discharge lamp of claim 14, in which a metal sheet (18)that is in superficial thermal contact with the discharge vessel hasrecesses that define ribs (19), and a middle region (21) connected to aperipheral region (20) of the metal sheet (18) via the ribs (19) has acooling device or is connected to a cooling device.
 16. The dischargelamp of claim 4, in which the discharge vessel (1, 2) is elongated inbarlike form.